I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to determining transmitted or received data rates.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and TIA-EIA-95. In particular, TIA-EIA-95 and its derivatives, IS-95A,IS-95B, ANSI J-STD-008, and future derivatives and enhancements, etc. (often referred to collectively herein as IS-95), are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459, which is assigned to the assignee of the present invention and fully incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-link, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MUTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same RF band. For example, each mobile subscriber unit (typically a cellular telephone) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum. It is to be understood that while 1.25 MHz is a preferred CDMA channel bandwidth, the CDMA channel bandwidth need not be restricted to 1.25 MHz, and could instead be any number, such as, e.g., 5 MHz.
Transmitting signals over the same RF spectrum provides various benefits including, e.g., an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. (In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station.) An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In conventional cellular telephone systems, a public switched telephone network (PSTN) (typically a telephone company) and a mobile switching center (MSC) communicate with one or more base station controllers (BSCs) over standardized E1 and/or T1 telephone lines (hereinafter referred to as E1/T1 lines). The BSCs communicate with base station transceiver subsystems (BTSs) (also referred to as either base stations or cell sites), and with each other, over a backhaul comprising E1/T1 lines. The BTSs communicate with mobile units (i.e., cellular telephones) via RF signals sent over the air.
In conventional systems, base stations, or cell sites, are configured to communicate via an over-the-air interface with various mobile units. In CDMA cellular systems, the base stations (sometimes referred to herein as base station transceiver subsystems (BTSs)) are often segmented into sectors, as defined by directional antennas, to increase the capacity of the cell. The sectors themselves may be referred to as cell sites. Conventional base station architectures typically employ three such sectors, with the radial directions each sector antenna points differing by 120 degrees. Each sector in a CDMA system functions, for network purposes, as an independent base station.
Previous CDMA based systems have used variable rate vocoders in a transmitting unit. The units are capable of transmitting at one of several predetermined frame rates. The intended receiver in these systems must determine which of the possible frame rates has been transmitted. The rate decision is determined by the blind rate determination algorithm (RDA) implemented within the receiver, which classifies each frame based on several frame parameters. The output of the rate determination operation is an indication of the likely sent frame rate along with an indication of whether or not an error is present within the received frame.
Previous rate determination methodologies have included using Cyclic Redundancy Check (CRC) bits; re-encoded symbol error counts; and Yamamoto quality bits, either alone or in combination.
Typically, all frame rates are decoded before making the decision as to which rate was actually transmitted. This brute force methodology utilized scarce resources in a non-optimal manner. Previous methodologies also encountered performance problems.
What is needed is an apparatus and method for optimally determining the frame rate at the receiving end of a transmitted signal and to improve the performance of the rate decision methodology.
The present invention is directed to a system and method for determining a received data rate in a radiotelephone system. The apparatus includes a correlator for accumulating a received signal representative of received energy levels to form an accumulated signal. Additionally included is a comparator for comparing an accumulated signal to a predetermined reference level and generating a second signal indicative of a particular data rate, wherein the data rate is either full, half, quarter or eighth rate and is indicative of the received energy level.
An embodiment of the invention includes a method for determining a data rate in a radiotelephone system with the steps of receiving a signal, receiving a signal transmitted from a mobile station at a predetermined data rate, combining the received signal in a RAKE receiver into a summed signal, and then comparing the summed signal with a predetermined threshold to form a comparison result. The most likely data rate is then based on the comparison result.